Lab 3 - Gaurav Singh Brooke Leslie/Chad Landrie 18 February...

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Gaurav Singh Brooke Leslie/Chad Landrie 18 February 2009 Lab Three: Separation of Liquids by Simple Distillation, and Analysis by Gas Chromatography Introduction The purposes of this lab were to set up a simple distillation apparatus, separate two liquids (a 1:1 mixture of ethyl acetate and butyl acetate) by simple distillation, properly use the GC (gas chromatography) machines, determine the percent composition of each of the two liquids in each fraction, and to construct a volume versus temperature graph during the distillation to indicate when to switch fractions. Simple distillation involves the separation of distillates from substances that are less volatile, which remain as residue at the completion of the distillation. This method allows for the separation of the components of the mixture if the difference between the boiling points of the two is greater than 40-50 degrees Celsius. A nonvolatile impurity in a pure liquid (such as sugar in water) reduces the vapor pressure of the liquid because it lowers the concentration of the volatile constituent in the liquid phase. The presence of the nonvolatile impurity reduces the vapor pressure at any temperature by a constant amount as can be discovered by Raoult’s law. As a result, the boiling point is higher. Although there is a nonvolatile impurity, the head temperature in the distillation is the same as for pure water (760 torr, 100 degrees Celsius), but the pot temperature will be elevated because of the decreased vapor pressure of the solution.
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Raoult’s Law applies here because it is the quantitative relationship between vapor pressure and composition of homogenous liquid mixtures. Px is the partial pressure of component x, and it equals the vapor pressure of pure x times the mole fraction of X in the mixture. The law only applies to ideal solutions, in which the interactions between like molecules are the same as the interaction between unlike molecules. When a mixture contains two or more volatile components, however, the total vapor pressure is the sum of the partial vapor pressures of each component (Dalton’s Law). Ethyl acetate has a boiling point of 77 degrees Celsius, and it is therefore distilled before butyl acetate, which has a boiling point of 126 degrees Celsius. As the process of condensing and vaporizing continue, the vapor becomes more and more concentrated in the more volatile component. It is important to know, however, if the distillation indeed occurred successfully. This is done using gas-liquid chromatography. This technique works on the idea of partitioning the portions of a mixture between a mobile gaseous phase and a stationary phase that is liquid. A sample is injected into a heated chamber where it is vaporized immediately and then carried by a flowing inert gas (which is appropriately named the carrier gas). The column is chockfull of finely divided solid support that has a high boiling liquid called the stationary phase. As the
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Lab 3 - Gaurav Singh Brooke Leslie/Chad Landrie 18 February...

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